1. Field of the Invention
This invention relates generally to a power grid system and, more particularly, a system and method for measuring a parameter of an alternating current power grid while minimizing the likelihood of lightning damage to a measuring system.
2. Description of the Background Art
Various devices measure parameters on an alternating current power grid. These devices may be located on infrastructure within the power grid or in individual homes and businesses attached to the power grid.
Some measuring devices measure a parameter on the power grid relative to a signal received via an antenna. For example, Phasor Measuring Units (PMUs) and Micro Phasor Measuring Units (μPMUs) may measure voltage phase angles relative to a Global Positioning System (GPS) timing signal received from GPS satellites. The GPS timing signal is received through an antenna, which may be exposed to the sky in order to maximize signals from the GPS satellites. This exposure increases the risk of damage by lightning.
To minimize the risk of lightning damage to a PMU or other measuring system, the antenna and corresponding signal receiver may be electrically isolated from the rest of the measuring system. Typically, in prior art, the antenna is electrically coupled to the signal receiver, and the signal receiver is coupled to the PMU or other measuring system via a signal cable. In prior art this signal cable may be electrically isolated at either the receiver end of the cable or at the measuring device end of the cable. These configurations reduce but do not eliminate lightning problems. For example, the cable between the antenna/signal receiver and the measuring device may be tens or even hundreds of meters long (e.g., if the antenna and receiver are on the roof of a tall building and the measuring device is in the basement of the building) and, even with electrical isolation at one end of the cable, the cable still provides a long electrical path and may still act as a lightning rod. As will be recognized by one skilled in the art, coupling a cable with a long electrical path to a signal receiver may degrade the receiver's signal-to-noise ratio, and coupling a cable with a long electrical path to a measuring device may cause undesired signals to enter the measuring device, commonly radio frequency signals from nearby transmitters and other sources.
Furthermore, electrical isolation and long cables between the signal receiver and the measuring system introduce time delays in the signal. (For timing signals with accuracies on the order of tens of nanoseconds, for example, cable delays become important when the cables are on the order of ten meters long.) The extent of the delay may vary from location to location, especially as the distance from the antenna to the receiver, and the distance from the receiver to the measuring device may vary from location to location. This can reduce timing accuracy when measurements at two different locations need to be synchronized precisely and compared, as in the case with PMUs. Therefore, there is a need for a system and method that enables accurate measurement of a parameter of an alternating current power grid despite timing signal delays, while also minimizing the likelihood of lightning damage to the system.